Otitis media is one of the largest public health problems of young children in the United States, and also contributes to half of the world's burden of hearing loss and 28,000 annual deaths worldwide. Otitis media is thought to be a multifactorial condition that can be induced by a variety of inciting events. However, once initiated, otitis media often converges upon a final common pathway of inflammation, effusion and tissue hyperplasia that in turn can produce temporary or permanent hearing loss. Upon resolution of otitis media the hyperplastic middle ear mucosa can recover to a condition at or close to its original structure, although permanent changes including fibrosis and osteoneogenesis can occur. The long-term goal of this project is to understand host responses in the middle ear during otitis media, with a particular focus on the transformation of the middle ear mucosa from a resting state into a highly active, hyperplastic structure that contributes to both host defense and pathogenesis. During the current period of support we have discovered two novel mediators that play significant roles in mucosal hyperplasia and inflammation. Based on our observation of the properties of these two mediators in the middle ear, we propose a new model of host responses in otitis media. In this model, the transmembrane tissue gowth suppressor and sentinel protein ECRG4 inhibits mucosal hyperplasia and inflammation in the normal middle ear. Bacterial infection induces cleavage of the exracellular domain of ECRG4 to release an 8 kDa fragment. Cleavage inactivates full-length ECRG4, releasing the mucosa from growth inhibition, and also allowing production of the leukocyte extravasation factor CD44. In addition, the dual-function 8 kDa fragment induces mucosal tissue hyperplasia and also binds to the TLR4/MD2/CD14 endotoxin receptor complex to enhance its activation. Activation of NFkB by the receptor induces expression of HB-EGF, which also further induces mucosal hyperplasia. We will test this model by manipulating the expression of ECRG4, CD44 and HB-EGF. The ultimate goal of this research is to identify new targets for pharmacological manipulation that will reduce otitis media pathogenesis and enhance recovery.